The 33-year-old originally had planned to make his first Minor League rehab start Tuesday night for Triple-A Tacoma, but the threat of rain led the Mariners to keep him at Safeco Field and throw under the roof prior to Tuesday night's game with the Astros.

Iwakuma said he felt much sharper than in his first simulated outing last Friday in Miami, and there's a chance he could be ready to join the Mariners after a rehab start in Las Vegas this Sunday for Tacoma.

If all goes well, he could be in line to return at some point during Seattle's nine-day, 10-game trek to New York, Houston and Oakland from April 29-May 7.

Manager Lloyd McClendon said Iwakuma will throw about 75 pitches in the Las Vegas outing, but continued to stress caution as the Japanese standout builds his arm strength up after missing all of Spring Training with a sprained tendon in his right middle finger.

"We'll see how it goes," McClendon said. "You have to be patient because we're talking about a guy that isn't a one-year wonder. I plan on this guy being around a long time because I plan on being around a long time. I want to make sure I take care of him."

"Nasty," said catcher John Buck after working the four innings behind the plate.

"It felt a lot better," Iwakuma said through translator Antony Suzuki. "The feel for the game is coming back gradually. I felt a lot better than the last time we did the sim game in Miami. Everything is moving forward. The ball jumped out of my hand pretty well. I feel pretty close now to the regular season."

Waits said the 33-year-old continues to have no problems with his finger and the focus now is strictly on building up his arm strength and conditioning.

"The main thing is he's getting better each time, from each bullpen to each sim game," said Waits. "The thing I was most impressed with today was in his fourth inning he still had great arm strength. That's what I was looking for. He wasn't tiring. He probably needs to work a little more from his full windup to get his timing, but all four pitches were working."

Iwakuma finished third in the American League Cy Young voting last year after going 14-6 with a 2.66 ERA in 33 starts and would be a welcome boost for a Seattle rotation that also has James Paxton, Taijuan Walker and Blake Beavan on the disabled list.